The kinetics of the reaction of goethite (alpha-FeOOH(s)) and barium hydroxide (Ba(OH)2) in an alkaline solution to produce BaO.6Fe2O3 by the hydrothermal method was investigated in the present study. The reaction was carried out in a batch closed reactor under high temperature and high pressure. The effects of alpha-FeOOH(s), Ba(OH)2 and NaOH concentrations in the feed and the temperature on the reaction rate were examined. It was found that the reaction rate was highly dependent upon the concentration of Fe(OH)-(aq) (i.e., the dissolved alpha-FeOOH(s)) and Ba(OH)2. A kinetic model, based on the homogeneous phase reaction, was built to describe the dissolution of alpha-FeOOH(s) and the precipitation of the dissolved alpha-FeOOH(s) (or Fe(OH)4-(aq)) and Ba(OH)2 in sequence to produce the desired product, BaO.6Fe2O3. The reaction rate at 280-degrees-C is expressed as function of Fe(OH)4-(aq) and Ba2+ concentration, i.e. R(p) = 0.19[Fe(OH)4-(aq)]2[Ba2+]/1 + 8.6[Fe(OH)4-(aq)]